376 



THE POPULAR EDUCATOR. 



6J inches high by 3 inches in diameter. It is arranged for 

 paraffin or gas. 



Figs. 2 and 3 represent other and even more convenient lamps 

 to be nsed with Reade's illuminator. In November, 1869, 

 Dr. S. W. Eoyston Pigott, M.A., M.D. Cantab., contributed a 

 valuable paper to the Royal Microscopical Society, on " High 

 Power Definition," with illustrative examples, in which many 

 learned suggestions were made, none of which shows more clearly 

 the restless nature of 

 scientific progress and re- 

 search than the following 

 prophecy, which seems to 

 threaten the great world 

 of mioroscopists with a 

 revolution that may set 

 aside some of the most 

 refined " glasses " at pre- 

 sent constructed : 



" The extraordinary dif- 

 ference between the per- 

 formances of the hydro- 

 objective and of the 

 pneumo- objective (the 

 plate of air or water mak- 

 ing enormous differences 

 in the aberrations of the 

 glasses) must make it 

 apparent to ordinary com- 

 mon sense that our old- 

 fashioned glasses are 

 wrong somewhere ; and if 

 not in failing to con- 

 verge the image of one 



point to another point, I know not where to find it, i.e., in 

 aberration, chromatic aberration being more easily com- 

 pensated. I know it is very difficult to throw aside the creed 

 and belief of forty years, and I have hesitated a long time 

 to bring forward my views, being perfectly convinced that 

 there would be a Battle of the Glasses to be fought ; and 

 the manner in which the subject of aberration has been 

 treated amply justifies my apprehensions. I point to a 

 well-known lens as an irrefragable proof of the deficiencies of 

 the corrections of old-fashioned glasses to grapple with some of 

 the exquisite difficulties of microscopic research ; and if my poor 

 effort shall in any way 

 advance the excellence of 

 defining power, especially 

 in the higher range of 

 investigations, I shall in 

 the end feel amply re- 

 warded. The work has 

 been earnest and sin- 

 cere." 



Enough has been said 

 to indicate some of the 

 " optical difficulties " to 

 be discovered in the path 

 of the very refined mi- 

 croscopical investigator, 

 either in the imperfect 

 illumination of the ob- 

 jeefc, or the glasses by 

 which the latter is to 

 be examined. However, 

 plenty of satisfactory and 

 delightful amusement re- 

 mains for those who are 

 content to follow in the 

 beateii track ; contrivers 

 of popular apparatus 



tread upon each other's heels until all seems to be made as 

 clear as the light of day in the enjoyment of our optical 

 pleasures. 



As an illustration of the pretty optical recreative toys in- 

 vented in recent years may be instanced the apparatus called 

 the " Photoscope," which may, we believe, be procured of all 

 dealers in optical instruments. It consists of a conical tube, 

 of which the base corresponds with a screen or disc, and is 



Fig. 2. 



represented by ground glass, and the truncated apex of the cone 

 is formed by the microscopical lenses and focussing arrangement. 

 Fig. 4 shows the whole apparatus in elevation. A B is the conical 

 tube, having the ground glass at the letter B, and the micro- 

 scope slide-stage at A. In order to illuminate the object, the 

 end of the instrument may be furnished with a reflecting mirror 

 like the Hollander's apparatus already explained at page 249. 

 A light tin conical tube, T, and open at the bottom, is fitted 



over the eye-glass at E, 

 when it is desired to pro- 

 ject an image of the ob- 

 ject in the slide on the 

 ground glass at B. The 

 instrument is either held 

 in the sun's rays so that 

 they impinge upon the 

 attached mirror, or if the 

 photoscope is shown at 

 night, a piece of magne- 

 sium band is ignited at 

 M ; and whilst this bril- 

 liant light is obtained 

 at M, the figure becomes 

 visible at B if the lenses 

 are carefully brought to- 

 focus by the lever at L. 



In Fig. 5 the instru- 

 ment is turned round so 

 as to show the figure 

 (not a rara avis) on the 

 ground-glass screen at B. 

 Of course some practice 

 is required in using the 



magnesium band, and in placing it in the right position. It 

 would be better to use a fixed magnesium lamp, which would 

 prevent the unsteadiness arising from the movement of the- 

 hand. 



The lenses used in this contrivance are shown in Fig. 6. 

 The lens nearest the eye, or that to which the burning mag- 

 nesium band is held, is a double-convex, and is shown at A. The 

 next is B B, the stage to carry the object, shown by the dotted 

 line, c is a very small double-convex lens, of a short focus,, 

 which is moved towards A, or away from it, by the lever D. 

 The diaphragm to reduce aberration is shown at E E, and being 



a very small aperture, it 

 shows what great care 

 must be taken to get 

 the light exactly in the 

 centre of the first lens, 

 or condenser, at A. 

 This optical arrangement 

 ia screwed on to the 

 conical body, F F, which, 

 as already stated, has a, 

 circular piece of ground- 

 glass fitted into the end 

 B (Fig. 4). 



Fancy might compare 

 the camera-like photo- 

 scope to the human eye, 

 and here we reach the 

 pinnacle of beauty in 

 that wondrously perfect 

 refracting organ. It 

 matters not where we 

 commence the investiga- 

 tion of this portion of 

 the human body : if, with 

 the poet, we take merely 

 the result produced by 



the eyes as one of the signs of expression of the mind, we find 

 not a word spoken ; only 



" Her lips were silent, scarcely beat her heart ; 

 Her eyes alone proclaimed we must not part." 



It is unnecessary to repeat that which has been so often spoken 

 of, namely, a description of the eye ; it is like reading Bradshaw's 

 Guide to take the student-traveller through a long description 



